1. Field of the Invention and Related Art Statement
The present invention relates to a circuit for measuring a diffusion-limited current for use in an oxygen concentration sensor and the like.
An oxygen sensor for measuring an oxygen concentration in a gas to be measured has been widely used. In such a sensor the atmospheric air defining a standard or reference oxygen concentration is brought to one surface of a cell body made by zirconia ceramics, the measured gas is brought to the other surface of said cell, and a given bias voltage is applied across the cell body. Then, an amount of current passing across the cell body is proportional to the oxygen concentration in the gas to be measured. Therefore, by detecting the current, it is possible to measure the oxygen concentration. As shown in FIG. 1, the current flowing across the zirconia body becomes substantially constant within a given range of the bias voltage and the current is dependent solely upon the oxygen concentration. FIG. 1 is a graph showing a variation of the current by taking, as a parameter, oxygen concentrations from 1% to 4%, while the bias voltage is applied across electrodes provided on the surfaces of the cell body. The current is increased in proportion to the bias voltage within a range from zero voltage to a given voltage. Thereafter, the current remains substantially constant even though the bias voltage is increased in a range from said given voltage to a specific voltage. This region is called the diffusion-limited current region. When the bias voltage is increased beyond the specific value, the current increases again in proportion to the applied bias voltage. This region is called the ion conduction region. That is to say, under a given bias voltage condition, the current is limited predominantly by the ion diffusion. Thus, in general, such a current is called the diffusion-limited current and the cell producing such a current is termed the diffusion-limited current element.
FIG. 2 is a circuit diagram showing a known circuit for detecting the diffusion-limited current. A diffusion-limited current element 1 constructed in the form of a tube has an inner electrode 1a connected to
ground and an outer electrode 1b connected to an output resistor 2 for deriving an output signal. The output resistor 2 is connected via a current driving transistor 3 to a supply source voltage V.sub.s. A base electrode of the driving transistor 3 is connected through current limiting resistor 4 and buffer amplifier 5 to a potentiometer 6 whose one end is connected to a point between a Zener diode 7 and a resistor 8 and the other end being connected to ground via a resistor 9. A series circuit of the Zener diode 7 and resistor 8 is connected across a voltage supply source +V and ground. Further, a diode 10 is connected between the base and emitter of the driving transistor 3. In this known circuit, to the base of driving transistor 3 is applied a constant reference voltage produced by the Zener diode 7, so that a constant voltage is applied across the series circuit of the diffusion-limited current element 1 and output resistor 2. Therefore, across the output resistor 2 a voltage drop is produced which is proportional to the diffusion-limited current flowing through the element 1. Therefore, by monitoring the voltage drop across the output resistor 2, it is possible to measure the concentration of oxygen contained in the gas which is brought into contact with the outer surface of the element 1, while the inner surface of the element is brought into contact with the reference gas defining the reference oxygen concentration.
In the above explained circuit for measuring the diffusion-limited current, when the diffusion-limited current I flows through the element 1, the voltage applied across the element is decreased by an amount equal to I.R, wherein R is a resistance of the output resistor 2. Therefore, the bias voltage applied across the element 1 is varied as shown by an inclined line B, even if the constant bias voltage V.sub.B is applied across the series circuit of the element 1 and output resistor 2 as illustrated by a vertical line A in FIG. 1. Then, the precision of measurement is decreased and a measurable range of the oxygen concentration is limited. In order to avoid the above drawbacks, in the known circuit the voltage drop across the resistor 2 is made small. For instance, the resistance R of the output resistor 2 is chosen such that the maximum voltage drop across the output resistor does not become higher than about 0.3 V. Therefore, an amplitude of the output signal, i.e. the voltage drop across the output resistor 2 becomes extremely small and S/N of the current signal could not be made high.